The present invention relates to DC-DC (Direct Current-Direct Current) converters and more particularly to a snubber circuit and a miniaturized switching circuit to perform an increased-frequency and increased-efficiency operation, and also to computers including such a DC-DC converter for weight reduction.
Miniaturization is the most important subject in a DC-DC converter in which a DC voltage source, a load and switching elements are connected in series, power being supplied to the load by turning on and off the switching elements. One means effective for realizing miniaturization provides an increased-frequency circuit operation to thereby miniaturize passive parts such as ceramic parts, capacitors, etc.
One known example of a miniaturized DC-DC converter which is directed to an increased-frequency operation is disclosed in FIG. 1 in which reference numeral 3 denotes a DC voltage source; 41, 42, a source dividing capacitor; 51, 52, a MOS FET; 191, 192, a charging and discharging snubber; 18, a transformer; 91, 92, a rectifying diode; 11, an output smoothing reactor; 12, an output smoothing capacitor; and 33, a load.
Factors which hinder the provision of an increased-frequency operation and miniaturizing of the DC-DC converter will be described.
A first factor is switching loss which increases in proportion to frequency. Thus, as the frequency increases, a cooling fan for the switching elements becomes large-scaled to thereby render it difficult to miniaturize the converter. In the conventional art, a turn-off loss in the switching elements is reduced by connecting the switching elements, which are MOS FETs 51 and 52, with charging and discharging snubber circuits 191 and 192, each comprising a resistor, a capacitor and a diode, connected in parallel with MOS FETs 51 and 52, respectively, to thereby realize an increased frequency operation and miniaturization. In this example, there is the problem that the snubber circuits are not lossless and that the energy stored by the capacitors is consumed by the resistors to generate a large loss, so that the converter using such snubber circuits has resistors increasing in size as the frequency handled increases to thereby render it difficult to miniaturize the converter. In addition, the power source efficiency is low.
A second factor hindering an increased frequency operation and miniaturization of the converter is a decrease in the maximum on-duty due to the presence of a second side overlap interval. When the output smoothing circuit has a choke input type structure, as shown in FIG. 1, commutation on the secondary side of the transformer is suppressed and generated slowly by a leakage inductance in the transformer, a lead inductance, etc., directly after the switching elements are turned on, and hence the secondary side of the transformer is shortcircuited for a particular interval of time, which is referred to as a secondary side overlap interval during which no power is transmitted to the load in spite of the switching elements being on. In the DC-DC converter having a choke input type output smoothing circuit, the percentage of the secondary side overlap interval occupied in the operation period increases as the frequency increases to thereby reduce the maximum on-duty. Therefore, the number of turns of the transformer windings is required to be set to a low value in order to ensure the output voltage in the case of the minimum input voltage and maximum load current. As a result, the primary side conversion value of the load current increases to also increase the loss in the primary side circuit to thereby increase the size of the cooling fan, to reduce the density of attached parts and hence to render it difficult to achieve a reduced size converter. The above conventional technique does not contemplate such points, and an increased-frequency operation and miniaturization of the converter is difficult. This problem becomes larger as the frequency increases, as the voltage output is decreased, or as the current output increases.
A third factor is a decrease in the maximum on-duty due to the presence of a dead time for prevention of a short circuit of the power source. In a closed loop comprising direct current voltage source 3 and two switching elements 51 and 52, as shown in FIG. 1, the power source is short-circuited if the switching elements 51 and 52 are turned on simultaneously. In this case, there is a probability that an excessive current flows through the switching elements to thereby destroy the elements. In order to prevent such short-circuiting of the power source, a dead time is provided between two on-signals such that the switching elements are never rendered conductive simultaneously even if there are variations in the circuit characteristics including the switching element characteristics. Usually, the dead time is set to a value similar to the turn-off time of the elements. For example, in a 500V-30A class MOS FET, the dead time is required to be about 1 .mu.s. When the elements are operated at a switching frequency of 200 kHz using the FIG. 1 circuit, the dead time would occupy about 40% of a half period. Thus, the maximum on-duty decreases, so that the number of turns of the transformer windings is required to be reduced, as mentioned above, in order to ensure the output voltage at the minimum input voltage and maximum load current. As a result, the primary side conversion value of the load current increases, the loss in the primary side circuit increases, the cooling fan becomes large-sized, the density of attached parts is reduced, and miniaturization of the converter becomes difficult. The conventional techniques do not contemplate those points and hence an increased-frequency operation and miniaturization of the converter is difficult.
As described above, the main factors which hinder the high frequency operation and miniaturization of the DC-DC converters are an increase in the switching loss, a decrease in the maximum on-duty due to the secondary side overlap interval and a decrease in the maximum on-duty due to the dead time for prevention of source short-circuiting.